True fertility - the
percentage of hatching eggs which are fertile; this can only be determined by incubation,
candling and breakout of the clears to determine which eggs were fertile or by breaking
out potentially fertile eggs to examine the germinal disc (e.g. a sample might be examined
to estimate fertility of a flock).

Hatch of fertiles or Hatchability - the percentage of
fertile eggs which hatch (hopefully above 85%).

Hatcher - a machine used to maintain proper conditions
for embryos during the final few (usually three) days before hatching.

Incubator - a machine used to maintain proper conditions
for setting avian eggs.

Hatch, Percent hatch, Hatching percent, or Hatch of total - the
percentage of all eggs set which hatch whether they were fertile or not (a typical hatch
might be 80% to 90%).

Malpositions - an embryo in
any position except head under right wing positioned in the large end of
the shell. Examples of malpositions are head under left wing or head between legs.

Pip - an egg in which the chick has broken the shell in
an attempt to hatch. Sometimes chicks die shortly after piping the shell.

Positive development (PD)- these eggs
are candled out as clears because there is no blood formation. The germ was fertile
but died soon after cell growth resumed when the egg was warmed above 80oF.

Relative humidity - is a measure of the water vapor or
moisture in air. It can be determined from the wet-bulb and dry-bulb temperature
using a psychometric chart. Incubation instructions are sometimes give in wet-bulb
temperature or relative humidity.

Spread - the difference between Fertility
and Hatch (a 10% to 12% spread is typical for chicken eggs).

Wet bulb temperature - the temperature measured by a
standard thermometer equipped with a wet sock over the bulb. For accurate
measurements air must be moving over the wet sock to provide evaporation. A sling
psychrometer can be used to measure wet and dry-bulb temperature. Electronic sensors
are now available to measure humidity of air in incubators and egg storage rooms.